1. Field of the Invention
The present invention relates to a method for inspecting the surface of a moving strip, in particular of rolled sheet metal moving at high speed, for detecting surface defects, as well as to a system employing such an inspection method.
2. Discussion of the Background
In conventional types of surface inspection systems, in which the surfaces of a moving strip are examined automatically, a surface is inspected by forming at least one digital picture of at least one of the faces of the strip, these being made up of a set of successive rows of picture elements which are each assigned a digital value. The at least one digital picture is filtered in order to detect surface irregularities by detecting relative variations in the digital values, and the surface irregularities are processed in order to identify the type of surface defect corresponding to each detected irregularity.
According to this inspection technique, the surface irregularities are generally processed by identifying the defect, from a set of defects liable to occur on the surface being inspected, which corresponds to each irregularity. The analysis of the irregularities is thus carried out in the same way, irrespective of the nature of the detected irregularities.
This type of surface inspection system consequently has relatively low processing speeds, in particular because of the nature of the processing step, which requires a large number of relatively long and complex calculation operations.
The object of the invention is to overcome these drawbacks and to provide a surface inspection method which allows prior sorting of the detected surface irregularities to be carried out, irrespective of the nature of the surface being inspected.
It therefore relates to a method for inspecting the surface of a moving strip, of the aforementioned type, comprising the steps consisting in:
using photographic means to form at least one digital picture of at least one of the faces of the strip, the digital picture being made up of a set of successive rows of picture elements, each of which is assigned a digital value;
filtering the said at least one digital picture in order to detect surface irregularities by detecting relative variations in the said digital values; and
processing the said at least one filtered digital picture in order to identify the type of surface defect corresponding to each detected irregularity;
characterized in that, prior to the step of processing the said at least one digital picture, an overall characterization of the irregularities is carried out by determining, for each of them, the value of predetermined parameters characteristic of surface defects, and a prior classification of the said irregularities is carried out on the basis of the determined values of the said parameters, according to a set of predefined classes, the said processing step being carried out on each class.
Since the detected irregularities are classed beforehand according to a set of classes, on each of which the image processing is carried out, it can be seen that the latter is speeded up considerably with the aid of this prior trimming step.
Further, this prior classification makes it possible to reduce the incidence of recognition errors and therefore improve the quality of the identification.
The method according to the invention may furthermore have one or more of the following characteristics:
each predetermined parameter representing a general reference axis in a space whose dimensions correspond to the said parameters, regions which each correspond to one of the said predefined classes are delimited in the said space, prior to the said prior classification, each irregularity is represented in the said space by a point whose coordinates are the values of the said parameters, and the said prior classification is carried out by identifying the region to which each point belongs, and by assigning the corresponding irregularity to the class corresponding to the said region;
a second way of characterizing the irregularities is determined for each predefined class whose number of characteristic parameters is less than the number of characteristic parameters for overall characterization, and subsequent to the prior classification step, the value of the characteristic parameters of the second characterization method specific to the said class to which the irregularity belongs is determined for each detected irregularity, on the basis of the values of the characteristic parameters for overall characterization;
a simplified reference frame for representing the irregularities is determined for each region, the number of axes of which is less than the number of general reference axes, and subsequent to the prior classification step, a step of changing reference frame from the said general reference to the said simplified reference frame specific to the region to which the irregularity belongs is carried out for each irregularity represented;
the step of processing the irregularities includes a first step of identifying the defect corresponding to each irregularity, from a set of types of defects specific to the class to which the said irregularity belongs, and a second step of classifying the said identified defect with a view to confirming and refining the classification resulting from the said first classification step;
the method includes a step of qualifying the types of defects identified in terms of a first type of defects identified certainly and/or precisely and a second type of defects identified uncertainly and/or imprecisely, and in that the said second classification step is carried out only on the defects of type qualified as uncertain and/or imprecise;
the method further includes a step of grouping together identified defects using a set of predefined criteria, in particular geometric and/or topographical criteria;
the method furthermore includes the steps of counting the number of identified defects of the same type per unit length, and of comparing the said number of defects of each type with a predetermined threshold value representative of the minimum number of defects on the basis of which the said defects are liable to exhibit a periodic character, with a view to detecting periodic defects;
subsequent to the step of determining the value of the said parameters, and before the said prior classification step, a specific classification of the irregularities is carried out according to a set of elementary classes, and the population of the said elementary classes is analysed with a view to detecting periodic defects;
subsequent to the filtering step, in response to detection of a picture element of an irregularity, a storage zone for picture element rows which are successively delivered by the photographic means and include at least one picture element corresponding to at least one irregularity is defined in a memory, each storage zone is segmented into suspect zones each having at least one surface irregularity, suspect zones which are contained in successive storage zones and correspond to the same irregularity are paired, and the total number of rows of picture elements of the paired suspect zones is compared with a threshold for large-length defect detection, and if the said threshold is exceeded, the said step of processing the said at least one filtered digital picture is carried out only on one of the said paired suspect zones, the result of the processing being assigned to the other paired suspect zones.
The invention also relates to a system for inspecting the surface of a moving strip for implementing a method as defined above, characterized in that it has means for photographing at least one of the faces of the strip, a memory for storing at least one picture of the strip in the form of rows and columns of picture elements which are each associated with a digital value, a circuit for filtering the said at least one digital picture in order to detect surface irregularities on the strip, by detecting relative variations in the said digital values, and a unit for processing signals which is connected to the said filtering circuit and comprises means for calculating values of characteristic parameters of surface defects, means for classing the detected irregularities according to a set of clauses predefined on the basis of the values of the said parameters and means for identifying each irregularity from a set of types of defects liable to correspond to the said irregularity.